A year ago I wrote a story attempting to explain the perceptual science behind why some people looked at a picture of a woman’s dress and saw it as blue while others saw it as white. People really, really cared—not, I now know, because anyone gave a damn what color the dress actually was, but because its color seemed so manifestly obvious that when someone else saw it as something else, it felt like an affront, a threat to one’s self-image. Whoever disagreed with you wasn’t just wrong but, like, nuts, right? We’re all just one screenshot away from turning into a nation of Internet commenters, I guess.

It was a thrill, then, to use a scientific explanation (albeit, I’ll grant you, a speculative one) not to drive a wedge between opposing opinions but to unite them. I talked to some researchers who specialize in color vision, and they told me that the picture of the dress was a sort of optical illusion that worked through color instead of line and form. Owing to a property called color constancy—the brain’s tendency to interpret an object as having a specific color based on experience as opposed to the actual wavelength of light the object is reflecting—the dress could plausibly appear as more than one color. Unlike many optical illusions, though, most people didn’t flip back and forth between two interpretations. Once you saw the dress one way, you tended to keep seeing it that way.

Despite thousands of years of thinking and hundreds of years of research, scientists are still working out exactly how people see colors. The ineffable combination of photons bouncing off of objects in real space, entering people’s eyes, interacting with the pigments that talk to the optic nerve, converting into neuroelectrical signals, and then pinging around the visual cortex isn’t a solved problem. It’s on the tip of the spear of science, up there with unifying quantum physics and gravity, or figuring out how life first evolved on Earth.

What I didn’t write about last year, though, is the philosophical question of what color is and why we see it the way we do. You think the scientists have problems? Oy. As soon as you tell a philosopher that the subatomic particles that comprise all matter don’t have color as such, because photons don’t really interact with them in any meaningful way, or that the photons that bounce off of matter don’t make it past the back of the eyeball and instead transduce to electrical signals, manifested as images in brain-meat somehow … well, that’s how you blow a philosopher’s mind, my friend.

The big shots all took a crack at color—Aristotle, Aquinas, Galileo, Descartes, Newton, Goethe. Nobody could figure it out. And it’s a big deal because, as M. Chirimuuta writes in Outside Color: Perceptual Science and the Problem of Color in Philosophy (yes, this is going to be one of those kind of articles), “the color question is a single instance of the larger issue concerning the secondary qualities—the ontological status of smells, sounds, tastes, and the tactile qualities. It is assumed, rightly or not, that once we settle on a solution for color, this result will be smoothly translated across to our theories of these other modalities.”

In other words, color is a proxy for understanding the difference between objective reality and the version of it that people perceive with their senses and create in their brains. It’d be good to connect those things.

Well, don’t get your hopes up. As Chirmuuta explains, the way you think about color has a lot to do with the way you think about the universe. If you think that everything we perceive is just an outcome of how our brains convert inputs to neural zaps, you’re an antirealist. The Matrix is just as real as something “real,” if you see what I mean. If, on the other hand, you think it’s obvious that there’s a real world out there, and photons bounce off of things and their wavelengths have an objective meaning apart from perception, you’re a realist. Or maybe you’re a relationist, who thinks of colors “in terms of relations between objects and perceivers.” Color is maybe a “psychobiological” property that lets living things discriminate among objects acutely and reliably.

Is there color, in other words, without a brain to see it? What about animals with different visual systems, like the reptiles, bugs, and birds that can see wavelengths of the spectrum we humans can’t? Infrared and ultraviolet? Those colors are real to them and invisible—nonexistent—to people. But they still exist. Right? What about people who are tetrachromic, whose eyes have four ways of seeing colors instead of the usual three? Or people who are red-green colorblind, who perceive fewer colors than most humans? Just because some people see a given color and others don’t, that doesn’t make the color less real.

And don’t get me started on the shared-metaphor problem of whether your “red” is my “red,” and what those “reds” are “like.” Let’s just agree that when you and I see something with a wavelength of roughly 650 nm, it’s what we both call “red,” and be thankful for the consensual hallucination we construct as language.

Play a game with me. Imagine Earth before anything lived on it. It had water, rocks, and sky … but nobody was looking at it. The entire visible spectrum was there—white clouds, black night sky, green emerald crystals, yellow lightning flashes, red flowing magma. But none of the adjectives mattered in any real sense.

When terrestrial plants learned to pull energy from the broad-spectrum energy output of the sun, they did it by absorbing all the photons except the green ones. I’m simplifying here—quite a lot, actually—but the idea I want to get across is that plants are only accidentally green, a function of how photosynthesis happens, based on the molecular structure of chlorophyll (in roughly the same way that blood’s ability to carry oxygen via a molecule called hemoglobin, based around an iron atom, makes it necessarily, physically red.)

About 150 million years ago you get the first flowering, fruiting plants. So then you have a world where colors are a natural consequence of, and cue for, cyclical living processes like ripening, fertility, and death. Like the animals that came before us, we see colors because we need information, and colors—no matter what they are in a realist sense, encode things about what objects are doing. Colors aren’t adjectives. They’re verbs.

(Chirimuuta wants to call them adverbs, but the writer in me can’t abide that. Adverbs—yeesh.)

“Colors should not be though of as something belonging either to observers or objects,” Chirimuuta eventually writes. “Color vision is a joint project of the perceiver and perceived.”

And that actually brings me back to the dress. Because what made it something any of us cared about wasn’t what color the dress was, or had, or conveyed. It was how we talked about it. It was the “joint project,” as it were, of constructing color together, digitally, online, with the world.